1. Field of the Invention
This invention relates generally to nondestructive testing and assessment. In particular, the present invention relates to nondestructive testing and assessment of bonds and bond strength.
2. Description of Related Art
Accurate and effective nondestructive bond strength evaluation has been one of the more challenging and elusive nondestructive goals for decades. In certain aerospace sectors, bond testing has been done using an approach that determines the percent area covered by the bond. An assumption made by the percent area approach is that any area that is not covered by a bond has no strength and any area that is covered by a bond has full bond strength. Unfortunately, some “bonds” exist that are in physical contact, but have no strength. These are often referred to as “kissing bonds” and offer no strength but are measured by the percent area approach as fully strong.
The need for a technique to assess bond strength is significant. Bonds are an important structural element in many designs. A fully bonded structure is less costly to build, is lower in weight and offers improved fatigue properties over a similar structure with fasteners.
Aircraft structures are a good example of a critical geometry that benefits from full bonding. Since bonds are not sufficiently reliable, an aircraft structure uses both bonds and rivets to complete the structural assembly. For example, Lockheed Martin's KC-130 aircraft uses 500,000 rivets. A fully bonded structure without redundant rivets would have a significant cost and weight savings in addition to enhanced safety.
There are other geometries that cannot use redundant fastening. For example, solid-rocket motors use a bonded insulation to separate the burning fuel from the outer casing. Currently, there are no quantitative measurement systems to directly assess the bond quality of flight worthy components and quality assurance for such systems depends solely on process control.
Bonding is also an important part of commercial fabrication. Automotive systems use significant bonding during assembly. Furniture, sports equipment, and boating equipment all benefit from bonded assemblies. Bonding is a significant medical technique for some procedures. Yet, the technology for bond assessment is unable to verify strength in a given part.
Ultrasonics is one of the primary nondestructive approaches to assessing bond quality. Both pulse-echo as well as continuous wave resonance ultrasonic tests have been used to assess bond strength to varying degrees of success. Ultrasonic tests measure the reflected and/or transmitted wave energy that interacts with the bond. Typically, such tests determine geometric properties, such as voids. However, such tests cannot verify bond strength.
Thermography is another testing method that has had limited success. Similar to scanning ultrasonics, thermography cannot distinguish a kissing bond from a good bond.
An engineering approach to bond testing is spot sampling. Using this method, a component, or witness sample, is selected from a production line and tested to failure. The failure loads experienced by the witness sample are assumed to represent the failure load for all of the components that has been produced since the testing of the last witness sample.
Witness sampling has two major failings. First, all components since the last witness sample test are suspect and should be identified as such. That requires witness sample inventory and idle parts. Second, a specific witness sample might not accurately represent all of the output components.
A weak bond may be in use where a failure may be catastrophic. Therefore, witness sample techniques drive process costs to maintain very tight constraints on quality, a desired outcome, but at a cost that may exceed the return on investment when compared to the systems, methods, and apparatuses of this invention.